The present invention relates to web supporting and drying apparatus. In drying a moving web of material, such as paper, film or other sheet material, it is often desirable that the web be contactlessly supported during the drying operation, in order to avoid damage to the web itself or to any ink or coating on the web surface. A conventional arrangement for contactlessly supporting and drying a moving web includes upper and lower sets of air bars extending along a substantially horizontal stretch of the web. Heated air issuing from the air bars floatingly supports the web and expedites web drying. The air bar array is typically inside a dryer housing which can be maintained at a slightly sub-atmospheric pressure by an exhaust blower that draws off the volatiles emanating from the web as a result of the drying of the ink thereon, for example.
One example of such a dryer can be found in U.S. Pat. No. 5,112,220, the disclosure of which is hereby incorporated by reference. That patent discloses an air flotation dryer with a built-in afterburner, in which a plurality of air bars are positioned above and below the traveling web for the contactless drying of the coating on the web. In particular, the air bars are in air-receiving communication with an elaborate header system, and blow air towards the web so as to support and dry the web as it travels through the dryer enclosure.
Similarly, U.S. Pat. No. 5,333,395 discloses a drying apparatus for traveling webs which includes a cooling tunnel directly connected with the dryer, a combustion chamber for combusting solvent which becomes volatile during drying of the web, heat exchangers, etc.
U.S. Pat. No. 5,038,495 discloses a cooling device for cooling a web of material exiting a dryer. The cooling device comprises a substantially closed housing with an inlet and an outlet slit for the web of material. The housing includes a feed aperture at the outlet slit side for feeding outside air into the housing, and a discharge aperture at the inlet slit side for discharging air from the housing into the dryer. Air is fed through the housing counterflow to the direction of web travel. A series of nozzles bring the infed air into contact with the web of material.
Once the traveling web exits such dryers, it is often brought into partial wrapping engagement around a rotating roller or "chill roll" so that the web can have substantial intimate contact with the cylindrical surface of the roller for heat transfer purposes to rapidly cool the web. A problem that has persisted in connection with such processes is the tendency for a film of air to intrude between the web and the cylindrical surface of the roller, thereby inhibiting effective contact (and thus heat transfer) between them. It is known that a relatively thin "boundary layer" of air is picked up by the moving surfaces of the web and the roller and that some of this air becomes trapped in the wedge-shaped space where the web approaches the roller surface. Unless the web is under a relatively high lengthwise tension, or is moving lengthwise at a relatively low speed, the trapped air enters between the roller and the portion of the web that curves around it, forming a film between the roll and the curved web portion. It will be evident that where a web is to be heated or cooled by a roller around which it is partially wrapped, an insulating film of air between the web and the roller will materially reduce the efficiency of the heat transfer. In addition, where the prior drying operation is drying ink or some other coating that has been applied on the web, the air film that is carried with the moving web may result in solvent condensing on the chill roll surface. The result can be condensate marking, streaking, spotting and/or smudging of the printed web. At higher press speeds (dependent upon web tension and chill roll diameter), the accumulation (thickness) of the condensate film increases and may transfer to the printed web, thereby affecting quality and salability of the finished product. The accumulation and thickness of the condensate is associated with the air gap developed between the web and the chill roll surface, and results in the phenomenon of "web lift-off," a clearance gap between the web proper and the surface of the roll.
It therefore would be desirable to lower the bulk temperature of the web in order to decrease the heat load of the cooling or chill rolls. Lowered web bulk temperature also would decrease the evaporation rate of the solvent mixture coating the web, thereby reducing the visible vapors evolving from the web. Condensation that normally occurs at the dryer exit and on the cooling rolls could be controlled to a minimum, and the product quality of the web could be improved in view of the absence of excessive moisture loss from the web. Excessive moisture loss can cause deleterious curling or waviness of the web.